1. Field of the Invention
This invention resides in the field of ceramics, and incorporates technologies relating to nanocrystalline materials, carbon nanotubes, and sintering methods for densification and property enhancement of materials.
2. Description of the Prior Art
Ceramics that have a microstructure consisting of nano-sized crystalline grains, i.e., grains that are less than 100 nm in diameter, are known to have unique properties that set these materials apart from ceramics with larger-grain microstructures. As a result, nanocrystalline ceramics hold promise as high-performance materials for a wide variety of applications extending from microelectromechanical devices (MEMS) to materials of construction for heat engines, cutting tools, wear and friction surfaces, and space vehicles. Fulfillment of the promise of nanocrystalline ceramics has been limited however by the brittleness of these materials.
Among the various attempts to reduce the brittleness of nanocrystalline ceramics, the most prominent have been the development of composites in which secondary materials are dispersed throughout the matrix ceramic material. In some of the more recent developments, carbon nanotubes, specifically multi-wall carbon nanotubes, have been used as the secondary material. A description of “ceramic matrix nanocomposites containing carbon nanotubes” is found in Chang, S., et al. (Rensselaer Polytechnic Institute), U.S. Pat. No. 6,420,293 B1, issued Jul. 16, 2002 on an application filed on Aug. 25, 2000. While the description encompasses both single-wall and multi-wall carbon nanotubes, the only carbon nanotubes for which test data is presented in the patent are multi-wall carbon nanotubes. The description in the patent of the sintering of the starting powders to form a dense continuous mass is limited to hot isostatic pressing.
Single-wall carbon nanotubes possess extraordinary electrical conductivity as well as a thermal conductivity that is twice that of diamond. Thus, when single-wall carbon nanotubes have been added to polymer matrices, the nanotubes have been shown to give the polymer an electrical conductivity high enough to provide an electrostatic discharge. Other extraordinary properties of single-wall carbon nanotubes are mechanical properties such as stiffness (a Young's modulus of 1,400 GPa) and strength (a tensile strength well above 100 GPa). While the electrical properties have been successfully exploited, the mechanical properties have not. Iron-alumina composites that include carbon nanotubes, for example, have demonstrated only marginally higher fracture strength than alumina alone and markedly lower than carbon-free iron-alumina composites. Nor has there been much improvement in fracture toughness. The best results reported to date are those of Siegel, R. W., et al., in Scripta mater. 44 (2001): 2061-2064, in which a 24% increase in fracture toughness of alumina was achieved by nanosized alumina filled with multi-wall carbon nanotubes.
Of further relevance to this invention is the literature on electric field-assisted sintering, which is also known as spark plasma sintering, plasma activated sintering, and field-assisted sintering technique. Electric field-assisted sintering is disclosed in the literature for use on metals and ceramics, for consolidating polymers, for joining metals, and for crystal growth and promoting chemical reactions. The densification of alumina powder by electric field-assisted sintering is disclosed by Wang, S. W., et al., J. Mater. Res. 15(4) (April 2000): 982-987.